1. Field of the Invention
The present invention relates to a controller for an internal combustion engine and, more specifically, to a controller for a multi-cylinder internal combustion engine.
2. Description of the Background Art
In order to follow regulations on emission (emission of harmful gas) that have been made tighter these days, control of air-fuel ratio with higher accuracy has come to be required of an internal combustion engine. Particularly in a multi-cylinder internal combustion engine provided with a plurality of cylinders, imbalance of air-fuel ratio among cylinders (hereinafter also referred to as “A/F imbalance”) significantly degrades exhaust property and, therefore, air-fuel ratio control of still higher accuracy is desired.
In connection with the air-fuel ratio control of a multi-cylinder internal combustion engine, Japanese Patent Laying-Open No. 2002-266682, for example, discloses an air-fuel ratio controller for a multi-cylinder internal combustion engine in which variation in air-fuel ratio among cylinders is corrected if air-fuel ratio of a cylinder is varied from ratios of other cylinders.
For an internal combustion engine, an exhaust gas recirculation apparatus (hereinafter referred to as EGR) has been widely known, which recirculates part of exhaust gas in an exhaust manifold to an intake manifold, to reduce nitrogen oxide (NOx) and to improve mileage (see, for example, Japanese Patent Laying-Open Nos. 2005-325697, 2003-120380, H09-324683 and 2001-90598).
EGR, however, influences air-fuel ratio of air-fuel mixture supplied to the internal combustion engine and if there happens to be a defect (such as clogging) in the exhaust gas recirculating passage, the amount of recirculated exhaust gas varies, possibly affecting the air-fuel ratio control. Particularly in the multi-cylinder internal combustion engine mentioned above, a defect at a portion of exhaust gas recirculating passage communicated with intake manifolds of the cylinders causes A/F imbalance and, therefore, there is a concern that such a defect has significant influence on the air-fuel ratio control.
On the other hand, in the multi-cylinder internal combustion engine, generally, an injector for fuel injection is provided in each cylinder and the amount of intake air is not always uniform among the cylinders. Therefore, A/F imbalance may possibly results not only from EGR abnormality but also from other factors such as variation of air passage area among cylinders caused by deposits.
Therefore, it is not always simple to find the cause of A/F imbalance. Conventionally, when A/F imbalance occurs, it has been uniformly determined that normal operation of the internal combustion engine is impossible, and control for abnormal condition of the internal combustion engine has been executed, whereby the vehicle runs in a refuge mode. This limits performance of the vehicle after detection of A/F imbalance.
Further, in repair work, much man-hours have been spent to find the defective portion, lowering operating efficiency.